Identifying new drug molecules for treating human diseases is a time consuming and expensive process. A candidate drug molecule is usually first identified in a laboratory using an assay for a desired biological activity. The candidate drug is then tested in animals to identify any adverse side effects that might be caused by the drug. This phase of preclinical research and testing may take more than five years. See, e.g., J. A. Zivin, “Understanding Clinical Trials,” Scientific American, pp. 69-75 (April 2000). The candidate drug is then subjected to extensive clinical testing in humans to determine whether it continues to exhibit the desired biological activity, and whether it induces undesirable, perhaps fatal, side effects. This process may take up to a decade. Id. Adverse effects are often not identified until late in the clinical testing phase when considerable expense has been incurred testing the candidate drug.
For example, an agonist (also referred to as a full agonist) is a chemical substance that binds to a target molecule (e.g., a receptor molecule), in or on a cell, to produce a biochemical and/or physiological effect. A partial agonist is a chemical substance that binds to a target molecule, but does not produce as great a magnitude of a biochemical and/or physiological effect as the agonist. The maximum magnitude of the biochemical and/or physiological effect produced by an agonist of a target molecule cannot be produced by a partial agonist of the same target molecule, even by increasing the dosage of the partial agonist. Some agonists of a target molecule are medically useful drugs that typically produce both desirable and undesirable biological effects. In contrast, partial agonists of a target molecule, that are medically useful drugs, often produce a weaker undesirable biological response than does an agonist of the same target molecule. Thus, partial agonists may be better drugs than full agonists because a partial agonist causes a desirable biological effect, and causes little or no undesirable biological effects.
There is a need, therefore, for methods for identifying partial agonists of target molecules that possess a desirable biological activity, and which cause fewer, or less severe, adverse effects than an agonist of the same target molecules.